Dissovled Organic Ccarbon, (DOC's) certainly can be measured, but it's more a BOD test, you can also do a COD test as a substitute.
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In aquariums you can have VERY higher DOC ppms and zero algae.
It's not a set amount or a type specifically. It's more the draw upon the available O2 ppm in the aquarium.
If something is rotting in the plant tank, it'll suck a lot of O2 out. The RATE of the O2 removal to oxidize say, a rotting Aponogeton bulb, or dying roots, or a filter that's suddenly clogged(or a vacillation every week, few days, or monthly), or something that slows the growth down of the plants, which this vacillation can cause, these are things that lead to the algal issues for most cases, rather than some DOC ppm.
Take our friend the Tannins group, wide range of DOC's but does not cause algae. the Type of DOC's count and the Rate.
COD will help some, BOD as well but more complicated and I'm not sure BOD will be too helpful vs COD.
At the end of the day, DOC's are just a correlation, rather than a cause of algae and related strongly to poor, even if temporary/vacillating plant growth.
If you have a canister filter, clean the sucker more often. Changes in flow causes drops in the O2, less CO2, and poor plant health in many cases.
You can always do the default test for ANY ppm of DOC, nutrients etc. .....ye old Large and Frequent water changes.
That often times will not resolve the algal issues though. So lowered DOC does not make algae go away or cure the issue.
You could still argue that a rise in DOC's of some sort causes some algae blooms. But if the drop off is plant health, lower growth rates, rotting stuff, lack if filter flow, poor CO2/clogged diffusers........are NOT the causes(I would suggest those things are much more the root causes for issues), then perhaps. Liebig's law added other things like Temp, Flow(water in our case), CO2, light etc. Not just nutrients or DOC's. I mean you can hammer the DOC's, but why do you have them to begin with? At last the types that do cause algae? And the corollary to that is why doesn't the algae go after after you remove the DOC's? Probably because the root issue regarding plant growth has not been fixed. You can make assumptions about DOC's and algae all day long, does that fix algae or plant growth problems? Not really.
Does looking at how to get that old Plank bucket of Liebig's to fill up higher(Better plan growth) help? Virtually in every case.
I left for a little over 2.5 weeks, the CO2 ran out of course, water changes were done by a friend 2x a week, 70%. Oh you bet I got algae.
When I got back, the friend felt they had caused it all. I said no, my bad for not just swapping a new tank before going on Vacation. I'd say this is a pretty foul outbreak of one of the tougher types of algae to get rid of.
3 days later? Clean. Did not come back, I did a big water change, cleaned the plants, filter, added a new CO2 tank, then another water changes 2 days later just because and another one 3 more days later. 2nd pic is 3 days after fixing the issue. Now if your CO2 gas tank runs out and you do not catch that issue, it's pretty obvious reason why. But other reasons, like filter clogged, then that causes the CO2 to be not high enough etc, or the filter is clogged and going slightly anoxic(DOC source) and the reduced flow lowers the O2 ppms a bit, say 2 ppm, which is a lot. There are so many ways to nut something up. So unless things are growing well, and you want to purposely try to induce algae, it's much harder to see things clearly.
Why? You lack a "control".
